In order for nervous systems to function properly, the axons of developing neurons must project with precision to their targets. In vertebrates, Purkinje cells in the cerebellum project with remarkable precision onto their target neurons. Yet little is known about how Purkinje cells accomplish this precision. Immunocytochemical labelling of Purkinje cells in the corpus cerebelli of catfishes using anti-zebrin II reveals a uniquely accessible Purkinje cell projection. Axons arising from a subset of Purkinje cells gather into three fascicles that run along the lateral surface of the rostral hindbrain and project caudally to targets in two acousticolateralis nuclei. Developmental studies reveal the presence of ectopic axons in young but not older animals, and the addition of fascicles with age. Here in addition to using zebrin II immunocytochemistry, tract-tracing and transmission electron microscopy will be applied to expand these preliminary findings. In particular the developmental mechanisms underlying the specific features of this projection will be examined by testin the hypotheses that: 1) Purkinje cells achieve their precise connections by pruning exuberant and inappropriate axonal collateral; 2) fasciculation is important in sorting projecting Purkinje cell axons; and 3) there are spatial patterns of labelled projection Purkinje cells following injections of tract-tracers into individual target nuclei. Results will shed light on the developmental mechanisms used by a specialized, experimetally accessible Purkinje cell projection, and will be generalized, where appropriate, to less accessible Purkinje cells in other vertebrate cerebellums, including humans.